For harvesting, forest machines are known which move on a terrain by means of wheels. These include a harvester in which a harvesting device is provided at the end of a boom assembly, a so-called harvester head for cutting and felling a tree stem and sawing it to pieces of desired length. The sawed tree stems are collected by a loading apparatus, i.e. a forwarder equipped with a grapple, and transported in the load space.
Publication WO 89/00928 discloses a working machine that comprises two frame parts connected to each other by means of a joint. The joint is used for controlling the direction of motion of the work machine. The working machine travels along a curved line by folding frame parts, wherein the centre of turn and the pivoting axle are always at an intersection in which the directions that are perpendicular relative to the frame parts intersect, projected to the same horizontal plane. Said direction is at the same time the direction of the mutual rotation axis of each pair of wheels. The wheels are supported by suspension arms of a parallelogram mechanism, said arms keeping also a wheel vertical.
Forest machines may comprise a swivel base mounted on the frame to swivel around a vertical axis and equipped with a cabin and, by its side, a boom assembly provided with a harvester head at its end. In a manner known as such, the boom assembly can also be mounted in front of the cabin or on a separate swivel base apart from the cabin.
The application publication WO 92/10390 discloses a harvester comprising two frame parts and controlled by folding the frame parts. The wheels are supported by a pendulum arm, wherein the rotation axes of each pair of wheels do share the same direction, but are not always coincident, as the height of the terrain differs at different sides of the working machine. Thus, the rotation axes intersect even in four different locations when the frame is folded. Consequently, the actual swivel centre is vague and causes, in view of some of the wheels, also diagonal sliding in curves, which damages the terrain. Even under these circumstances the centre is always perpendicular in relation to each frame part. In prior art forwarders, the second frame part is provided with a load space, wherein the tree trunks are collected from the terrain by means of a rotatably attached boom assembly at the front part of this frame part and its grapple. The second frame part is equipped with a cabin and a prime mover for the working machine.
Application publication WO 99/710221 discloses a forwarder comprising three frame parts that are folded when driving in a curve, so that the rotation axes of all pairs of wheel would share a mutual intersection point. Forces directed to the frame joints are significant, particularly at the beginning of the curve, wherein the turning radius must be gradually diminished when moving, if an aim is to prevent the pair of front wheels from advancing diagonally. The wheels are also here suspended by means of swaying arms, which causes vagueness in the swivel centre as well as gliding. To increase the capacity of weight the load space is supported by two pairs of wheels, but in this case the load space must be supported on the frame parts by joints, because straight tree stems do not bend along with the frame parts. The structure is complex and when loaded it can even prevent the frame part from folding or at least increase the folding power needed in the frame joint.
The application publication DE 19822809 A1 discloses a load-carrying vehicle comprising two two-wheel axle-group structures in both frame parts to increase the capacity of weight. The wheels of the axle group sway around a common horizontal rotation axis, which is always perpendicular in relation to the frame part. In particular when driving in a curve with the frame folded, the rotation axes of wheels intersect at least at four different locations, wherein the wheels of some of the axle groups are always gliding diagonally, particularly when driving in a steep curve, which will damage the terrain. The axle group causes damage to a wider area because, when gliding, the successive wheels of the axle group have different turning radius due to the distance between the wheels.
One known forest machine is a so-called combined machine, which combines the functions of a harvester and a forwarder. In this case, e.g. the rear frame is provided with a load space and the boom assembly is mounted rotatably on the front frame. The boom assembly and the cabin can be placed on the same rotating swivel base.
The boom assembly is provided with a harvester head, which is applicable also for loading tree trunks, wherein it is provided e.g. with grapples. The advantage is that harvesting and collection of tree stems can be performed simultaneously, wherein the need for a plurality of different machines is reduced, which reduces damage caused to growing stock.
For example in forest thinning, several growing trees are left by the side of the logging road used by the working machine, which limits the space available for the working machine. To improve the movability and to bypass trees, the turning radius should often have to be as small as possible, which is, however, limited by the stability requirements of the machine. Thus, the working machine cannot be folded infinitely, because when curving, the inner wheels come closer to each other and the outer wheels diverge from each other, wherein the centre of gravity can be positioned adversely. This will increase the risk of falling, particularly if the working machine is additionally tilted or moving with a load.
To sum up, known forest machines share a limited ability to move on a very varying forest terrain and they cause damage to the terrain. Due to structures and balance, the turning radius are limited and space demanding, wherein unnecessary damage is caused also to growing stock. Nevertheless, the wheel must be able to move sufficiently in the vertical direction because of unstable forestry terrain and obstacles to be bypassed.
A particular problem is how to pass soft and squashy areas of the terrain. Particularly in 6-wheel or larger working machines and in heavy forwarders the successive wheels add to sinking of these spots and at the same time the swaying of the frame is increased. Simultaneously, the forces folding the frame joints also increase and the load distribution between the wheels can vary, even suddenly.